In military ordnance arts, carriers for projectiles, known as sabots, have been used to facilitate the use of a variety of munitions while engaging in military operations.
In general, a sabot is a lightweight carrier for a projectile that permits the firing of a variety of projectiles of a smaller caliber within a larger caliber weapon. A sabot provides structural support to a flight projectile within a gun tube under extremely high loads. Without adequate support from a sabot, a projectile may break up into many pieces when fired.
A sabot fills the bore of the gun tube while encasing the projectile to permit uniform and smooth firing of the weapon. The projectile is centrally located within the sabot that is generally symmetrical. After firing, the sabot and projectile clear the bore of the gun tube and the sabot is normally discarded some distance from the gun tube while the projectile continues toward the target.
One method for discarding a sabot is to form a scoop onto the sabot. After the sabot and projectile clear the weapon bore, the scoop gathers, or "scoops," air particles as it is moving forward. The air pressure on the front scoop lifts the sabot from the projectile and thus the sabot is removed from the projectile in flight, allowing the projectile to continue towards its target.
For a kinetic energy projectile supported by a sabot to have a high muzzle velocity, the parasitic weight around the flight projectile must be substantially minimized. Part of this objective has been achieved through the use of advanced lightweight graphite epoxy prepreg material for sabots. Prepreg is the material resulting from impregnating fiber reinforcements with a formulated resin. These advanced composite materials offer many advantages over conventional steel and aluminum since parts fabricated from prepreg materials are generally stronger, lighter and stiffer than metals. They also provide greater resistance to fatigue, creep, wear and corrosion than metals. Composite parts made from prepreg have very high strength in the direction of the fibers and very poor strength in other directions.
A composite sabot is typically fabricated from prepreg panels having plies oriented in different directions. A sabot's weight is substantially governed by its stiffness and strength in the axial direction since most of the loading is in axial direction. High axial strength and stiffness are often achieved at the expense of the stiffness in the radial direction.
In use, as a flight projectile travels down the flexible gun tube, it encounters significant lateral loads, called balloting loads. As the projectile bounces in the tube, the front bourrlet, or the scoop of sabot, undergoes deformation proportional to its radial stiffness. As the scoop bends under the application of lateral loads, the penetrator also bends and moves away from the tube centerline. The perturbations in the gun tube may result in the penetrator exhibiting a high yaw rate at muzzle exit. A high yaw rate causes poor target impact dispersion or accuracy. It is known that a stiffer front scoop improves the accuracy of projectile by reducing bending of the scoop.
Aluminum sabots maintain acceptable radial stiffness in the front scoop. Unfortunately, aluminum sabots suffer from the other drawbacks of metals noted above. Conversely, although the use of composite material for sabots has many advantages as listed above, prior known composite sabots exhibit poor radial stiffness as compared to aluminum sabots. Certain projectiles with aluminum sabots have proven very accurate. In contrast, a similar projectile with a conventional composite sabot does not compare favorably with the accuracy of a comparable aluminum sabot. It is believed that, since both projectiles use substantially the same kinetic penetrator, lower radial stiffness inherent in the conventional composite sabot contributes to the poor accuracy of the projectile using the conventional composite sabot.
Additionally, sabots are generally made in three symmetrical segments to facilitate smooth discard upon exit from the gun. Typically, each segment, or petal, spans 120 degrees of the front circumference of the intact sabot. The overall advantage of a three-petal sabot design is that the sabot is released more quickly, thereby reducing lateral disturbance to the flight projectile, thereby increasing accuracy.
Further, for optimal technical and marketplace performance, there are several other objectives to be considered when designing a sabot. For example, the sabot must be easy to build and cost effective. Further, the sabot must be lightweight, yet rigid and strong. Composite sabots are effective in obtaining most of these objectives; however, some aspects of rigidity and strength, in particular, radial strength, elude composite sabots.
Prior art weight reductions of composite sabots are made by aligning the prepreg fibers in the axial plane of the sabot which matches the greatest load directions generated during the projectile's travel down the weapon bore. This method of aligning all the fibers in the same direction throughout the sabot, to match the greatest loads, is commonly referred to as homogeneous composite architecture.
FIG. 4 shows an example of a homogeneous composite architecture 400 of the prior art developed by Alliant Techsystems Inc. used to make homogeneous architecture composite sabots. Illustrated in FIG. 4 is a top view of a homogenous layup 410 using homogeneous composite architecture 400. Homogeneous layup 410 comprises a panel including a plurality of homogeneous prepreg plies 412 stacked on top of each other. Further, homogeneous layup 410 is overlaid with a homogeneous layup pattern 408. Homogeneous layup pattern 408 arranges a plurality of homogeneous prepreg segments 450 Within homogeneous layup 410.
Each homogeneous prepreg ply 412 has a different fiber orientation, resulting in homogeneous fiber orientations 420. A first homogeneous fiber orientation 422 and a second homogeneous fiber orientation 424 are both oriented at 0 degrees with respect to a homogeneous sabot axial direction 440. A third homogeneous fiber orientation 426 and a fourth homogeneous fiber orientation 428 are not aligned with the homogeneous sabot axial direction 440, nor are they aligned with each other.
First homogeneous fiber orientation 422 and second homogeneous fiber orientation 424 create a dominant homogeneous fiber orientation 430 because they are aligned in the same direction. Dominant homogeneous fiber orientation 430 represents the direction in which homogeneous layup 410 has the most strength and rigidity. In this case, dominant homogeneous fiber orientation 430 aligns along homogeneous sabot axial direction 440.
All of the homogeneous prepreg segments 450 are also aligned along the homogeneous sabot axial direction 440. Hence, all of the homogenous prepreg segments 450 have the highest strength and rigidity along the homogeneous sabot axial direction 440. As a result, homogeneous composite architecture 400 provides a sabot with high axial strength and rigidity, but does so at the expense of lower radial strength and rigidity.
Lowering radial strength leads to poor accuracy, making homogenous composite architecture sabots less desirable than aluminum sabots. Additionally, as mentioned, the inadequate radial rigidity of the existing composite sabot scoops can lead to higher parasitic weight and lower impact velocity.
Another prior art technique called "tailored architecture" sought to overcome the problems with homogeneity by individually orienting each prepreg segment along the direction of dominant homogeneous fiber orientation to supply each part of the sabot with the required strength. Conventional tailored architecture uses a different layup for each prepreg segment. Unfortunately, using multiple layups creates a great deal of waste during manufacturing because only a few segments will be cut from each layup. Moreover, bookkeeping for all the different layups, orientations, and segments quickly becomes very difficult as the number of segments increases.
If segments are improperly aligned during fabrication, the result could be structural failure of the sabot. Sabot failure can cause a multitude of problems from weapon jams to misfires. Moreover, because advanced lightweight graphite epoxy materials are relatively expensive, the high cost of waste makes using prior art tailored architecture prohibitive.
In contrast to the prior art, the invention disclosed herein provides a simplified tailored architecture for use in fabricating composite sabots. The unique simplified architecture of the invention uses homogeneous composite ply panels to reduce cost and reduce the chance of misalignment of some critical segments during fabrication of kits. The simplified tailor architecture of the invention maintains high axial strength and stiffness necessary for resisting axial loads while providing high radial stiffness and strength in the front scoop and the rear bourrelet of the sabot.
Further in contrast to the prior art, the simplified tailored architecture of the invention features rotating the prepreg segments that comprise the front scoop and rear bulkhead in the direction of dominant homogeneous fiber orientation on the same layup that includes other segments aligned for high axial strength. Rotation of these segments does not affect kit or sabot segment molding processes. Orienting fibers in front scoop results in a significantly stiffer scoop to improve the yaw rate at muzzle exit.
Composite sabots built in accordance with the present invention have high scoop strength so that the sabot can be discarded faster. A stiffer front scoop and a faster discard rate yield a composite sabot having accuracy approaching that of an aluminum sabot, but without the drawbacks of using aluminum. Thus, the simplified tailored architecture of the invention preserves advantages of composite materials without adversely impacting the manufacturing process or cost of a sabot.